1. Field of the Invention
The present invention relates to hydraulic circuits for toroidal continuous variable transmissions (CVT) for vehicles in which frictional rollers are disposed between an input disk and an output disk such that rolling states of the frictional rollers are changed to thereby change a rotational speed ratio between the input and output shafts, and more particularly to improvements to a section for feeding a hydraulic pressure to the frictional elements which couple the input and output shafts for moving a vehicle forward or backward.
2. Description of the Prior Art
One of the conventional toroidal continuous variable transmissions is disclosed, for example, in Japanese unexamined patent publication No. 10-148244. This transmission includes frictional rollers called power rollers disposed in a toroidal groove formed between opposing surfaces of a pair of input and output disks disposed on the same shaft and supported rotatably by supporting mechanisms called trunnions. This supporting mechanism is driven by a hydraulic cylinder in a direction perpendicular to the axes of the frictional rollers and also to the common axis of the input and output disks. When the positional relationship between the axes of the frictional rollers and that of the input and output disks changes, the rotational direction of the frictional rollers and the rotational direction of the input torque applied from the input disk changes from their original ones, which causes axes of the frictional rollers to tilt between the input and output disks. Thus, a contact radius defined by a distance between each of the centers of the input and output disks and a corresponding one of points on the input and output disks where the respective frictional rollers contact with both the input and output disks changes to thereby change the ratio in rotational speed between the input and output.
Such hydraulic circuit for the toroidal continuous variable transmission is disclosed, for example, in Japanese unexamined patent publication No. 11-30317. Since the hydraulic circuit involves an automatic transmission, responsibility of the hydraulic pressure control involves that of speed change control. As well known, the responsibility of the hydraulic control changes depending on the viscosity of the hydraulic oil, which, in turn, depends on its temperature. A hydraulic control device for an automatic transmission which takes account of a temperature dependency of the hydraulic oil viscosity is disclosed, for example, in Japanese unexamined patent publications Nos. 2-42251 and 2-62468. In the former, the hydraulic oil pressure is set at a high value in order to improve the responsibility of the hydraulic oil at a low temperature since the viscosity of the hydraulic oil is high at the low temperature. In the latter, temperature sensing means senses the temperature of the hydraulic oil and the set hydraulic pressure is adjusted depending on its detected temperature in order to cope with the temperature dependency of the hydraulic oil viscosity.
The most important destination of the hydraulic pressure in the hydraulic circuit for the toroidal continuous variable transmission is a hydraulic pressure cylinder which drives the supporting mechanism to tilt axes of the frictional rollers. Since the hydraulic pressure cylinder supports a torque transmitted between the frictional rollers and the input and output disks, a proper substantial hydraulic pressure is required for that purpose. In the hydraulic pressure circuit for the automatic transmission, the highest adjusted hydraulic pressure is called a line pressure. In the case of the toroidal continuous variable transmission, the line pressure is set at a higher value than those in general automatic transmissions. In this case, the hydraulic pressures fed to other destinations are high as a whole, especially as long as the line pressure is not reduced. Thus, the hydraulic pressures are also high which are fed to frictional elements such as clutch elements which connect the input and output shafts to move the vehicle forward or backward.
Since the viscosity of the hydraulic oil is high at a low temperature, as described above, the line pressure is also set at a high value to ensure the required responsibility even in the toroidal continuous variable transmission. Thus, for example, if a travel range such as a D- or R- range is selected in a state where a non-travel range such as an N- or P- range is set when the vehicle is at a stop and the temperature of the hydraulic oil is low, a high hydraulic oil pressure is fed instantaneously to the frictional elements which connect the input and output shafts to move the vehicle forward or backward. Generally, in the case of frictional elements such as clutch elements, a volume of the cylinder chamber is set by allowing for a time required for relevant dish plates to come into complete contact with each other to transmit a torque to the output shaft. When high hydraulic oil pressure is fed instantaneously to the cylinder chamber, the cylinder chamber is instantaneously filled with the hydraulic oil, and the frictional elements are engaged in a short time to produce a high torque, which can be a shock.
It is therefore an object of the present invention to provide a hydraulic oil circuit for a toroidal continuous variable transmission in which even when a travel range is selected at a low oil temperature in a state where a non-travel range is selected, the frictional elements for moving the vehicle forward or backward are prevented from engaging with each other in a short time and hence high torque is prevented from being produced instantaneously.
In order to achieve the above object, accordidng to the present invention, there is provided a hydraulic oil pressure circuit for a toroidal continuous variable transmission for a vehicle in which a point where an input disk and an output disk contact with a frictional roller is changed to adjust a speed ratio, comprising: an input shaft and an output shaft coupled to the input and output disks, respectively; a pair of frictional elements for coupling the input and output shafts for moving the vehicle forward or backward; a source for supplying a hydraulic oil; a pressure control valve for receiving the hydraulic oil from the source and for adjusting a pressure of the hydraulic oil; oil path providing means connected to the pressure control valve for feeding therethrough the adjusted hydraulic pressure to the frictional elements; and a choke provided in the oil path providing means for imparting resistance to a flow of the hydraulic oil fed to the pair of frictional elements through the oil path providing means.
The choke serves to narrow a flow path to increase a flow resistance. In this case, the choke is preferably set such that a ratio of a length of a flow path to its diameter is more than 2. Thus, the flow is reduced in a low temperature area compared to a choke including a short orifice.
The choke is preferably provided upstream of an accumulator which stores the hydraulic oil pressure fed to the frictional elements.
A check valve is preferably provided in parallel with the choke for allowing only a hydraulic oil returning from the pair of frictional elements to pass therethrough.